A vehicle includes a power transmission apparatus for converting and outputting a torque of an internal combustion engine according to a driving condition. The power transmission apparatus includes a gear-type power transmission apparatus, a belt-type power transmission apparatus and the like. The gear-type power transmission apparatus, in which power transmission loss is small, is widely used.
The gear-type power transmission apparatus shifts gears (second power transmission members) for establishing a plurality of shift stages in order to output the inputted torque of the internal combustion engine according to the driving condition. The power transmission apparatus selects one of the gears for establishing the plurality of shift stages in order to shift to the selected gear.
The power transmission apparatus includes a synchronizer ring in order to perform a shift operation quickly and easily when the gears are shifted. The synchronizer ring includes a cup-shaped inner circumferential surface, formed into a taper-shape, which is engageable with a taper-shaped outer circumferential surface of a conical portion of the gear (alternatively, the cup-shaped inner circumferential surface is engageable with a taper-shaped outer circumferential surface of a gear piece, which integrally rotates with the gear). When rotation is synchronized in a manner where the outer circumferential surface of the conical portion of the gear and the cup-shaped inner circumferential surface of the synchronizer ring frictionally engage with each other, a synchronizing operation of the power transmission apparatus is completed, and then the gear becomes shiftable. A clearance is generated between the cup-shaped inner circumferential surface of the synchronizer ring and the outer circumferential surface of the conical portion of the gear in an axial direction and a radial direction of a rotational shaft, which is provided at the same side as axes of the synchronizer ring and the gear, so as to restrain contact therebetween when the synchronizing operation is not performed (i.e., when the synchronizer ring idly rotates). Therefore, when the synchronizer ring idly rotates, the synchronizer ring may be inclined due to rotation and its own weight. Because the gear rotates relative to the synchronizer ring, the cup-shaped inner circumferential surface of the inclining synchronizer ring may contact the outer circumferential surface of the conical portion, thereby causing a generation of dragging torque. When the dragging torque is generated, torque loss may occur, which decreases transmission efficiency. Further, the contacting surface may wear out and may be seized.
In order to restrain the contact between the cup-shaped inner circumferential surface of the synchronizer ring and the outer circumferential surface of the conical portion of the gear, various devices are introduced. For example, according to a synchronizer clutch mechanism, disclosed in JP2006-57717A (referred to as reference 1 hereinafter), generation of dragging torque due to contact between a cup-shaped inner circumferential surface of a synchronizer ring and an outer circumferential surface of a conical portion of a gear is restrained, in a manner where the synchronizer ring is stably held by means of a spring element attached to the synchronizer ring. Further, a synchromesh mechanism for a transmission, disclosed in JP2007-292151A (referred to as reference 2 hereinafter) includes a piece member, arranged between an inner circumferential surface of a synchronizer ring and an outer circumferential surface of a clutch hub (a first power transmitting member) in a radial direction, contacting the inner circumferential surface of the synchronizer ring, and generating biasing force for biasing the synchronizer ring in a radially outward direction, by means of centrifugal force action.
According to reference 1, however, a shape of the synchronizer ring is required to be modified minutely in order to attach the spring element to the synchronizer ring. Further, the spring element is not easily assembled because the spring element generates spring force. According to reference 2, the piece member biases one side of the synchronizer ring in an axial direction, from an inner circumferential surface of the synchronizer ring in a radially outward direction. Therefore, the synchronizer ring is likely to incline relative to the axial direction, and accordingly, the generation of the dragging torque due to the contact between the cup-shaped inner circumferential surface of the synchronizer ring and the outer circumferential surface of the conical portion of the gear, may not be restrained.
A need thus exists for a power transmission apparatus, which is not susceptible to the drawback mentioned above.